The present invention relates to optical communication systems and more particularly to systems and method for transmitting using optical filtering to shape the spectrum of a modulated optical signal.
With continuing increases in data traffic, it is desirable to increase the carrying capacity of optical communication links. One way of doing this is to add new fibers. Where optical fiber communication links already exist, they are supplemented with new fiber along the entire link. This however is extremely expensive due to the need to dig along the length of the link to install the new fiber, the need to install optical amplifiers along the new fiber, and the fact that the optical link may extend over hundreds or even thousands of kilometers.
Modern high capacity links typically already employ wavelength division multiplexing (WDM) techniques where a single fiber carries multiple optical signals having different wavelengths. In one representative configuration, there are multiple optical signals carrying 10 Gbps signals spaced 50 GHz apart in the frequency domain. The modulation employed is amplitude modulation (AM) so that each signal occupies 20 GHz of spectrum.
It would be desirable to increase the capacity of this link by, for example, a factor of 4. It would further be desirable to increase the link capacity without changing the WDM channel plan in order to minimize any modification of optical amplifiers or passive optical components along the link. A way of accomplishing this would be to increase the data rate carried by each optical signal to 40 Gbps while maintaining 50 GHz spacing. However, if amplitude modulation is used, each signal will occupy at least 80 GHz of spectrum causing spectral overlap of the modulated optical signals. Increasing the spacing to accommodate the wider modulated signals would reduce the number of WDM channels that can be accommodated, greatly reducing any capacity increase that would otherwise be achieved. Another problem with the increased data rate is that chromatic dispersion is increased due to the broadened spectrum of each signal while the tolerance to chromatic dispersion effects is reduced due to the narrowing of the pulse widths of the modulated signals.
An alternative to amplitude modulation is the use of vestigial sideband (VSB) or single sideband (SSB) techniques where one of the two sidebands introduced by amplitude modulation is suppressed (VSB) or entirely attenuated (SSB). In SSB, the carrier is also attenuated. These techniques are more spectrally efficient than AM. Furthermore, they allow data rate to be increased with less impact from chromatic dispersion effects. Proposed optical VSB and SSB optical transmitters involve the use of extremely complex and expensive modulators that synthesize the suppressed sideband signals. Besides complexity and expense, another drawback is that optical amplification would be required to compensate for the insertion loss introduced by these modulators.
What is needed are more easily implemented systems and methods for generating suppressed sideband optical signals.